I was just wondering about the 'two-propeller stack' in the
attachment...I understand how more blades help absorb
HP...but would the configuration shown in the attachment
be: efficient? noisy? Otherwise desirable? Thanks.

Gene Kahn

I don't have a clue :) I do know the fewer the blades the
more the efficiency down to one.

The other thing I know is efficiency of the tip is a strong
function of the thickness of tip. That is one reason metal
props are better than wood props.

Paul Lamar

A number of years ago an ultra-light designer named Dale Kramer in
Canada experimented with stacking two 2-bladed short diameter props
creating a sort of "biplane" configuration without struts.  He was the
designer of the Lazair aircraft and had some high turning industrial
engines on his aircraft and wanted more thrust without having to use a
reduction unit.  I've never seen any results from his experiments.  I'd
be surprised if some of the modelers haven't tried something like
stacking three or four-bladed props.  I know that at least one of the
hovercraft types (sport) tried it.

George W

In bi planes it is common to space the the wings at least one
cord length apart to avoid negative interaction.

Slotted flap configurations are a little bit similar to
two prop blades only a few degrees apart. To function correctly
however the central axis's of the blades must be parallel
and the leading blade ahead of the other angle wise.
Spacing is critical.

This would be a way of increasing thrust at the expense of drag
or less efficiency and not more. Since the best way to improve
the efficiency of a wing or prop blade is to make it longer. The problem
with that, at a given forward speed and RPM, the tips go transonic
so blade tip thickness and tip cord must be made less. Tip drag is there
fore the bottom line. Also the blade twist must be such as to insure
the tip angle of attack is near zero at the top speed of the
aircraft. In WW II the trend was to hollow steel blades to allow
the tips to be thin.

If worst came to worst you could saw off your opponents tail :)

Paul Lamar Merry Christmas


This discussion reminds me of the unusual profile of the prop designed by Paul
Lipps.
http://www.eaa.org/experimenter/articles/2010-09_phantom.asp
I think there was a detailed article in Contact! magazine last year or so.
C. Smith